Weighing cells of the known state of the art operate according to different working principles based for example on strain gauges, on taut-string oscillators, or on electromagnetic force compensation (EMFC). Gravimetric measuring instruments with oscillating-string or EMFC weighing cells produce weighing results of a very high resolution.
In EMFC weighing cells, the weight of the load is transmitted either directly or by way of one or more force-transmitting levers to an electromechanical measurement transducer which generates an electrical signal representative of the weight of the weighing load. The signal is further processed by an electronic signal-processing arrangement and presented in an indicator display.
In their mechanical configuration, weighing cells that are based on the oscillating-string principle are largely analogous to EMFC weighing cells except that an oscillating-string transducer is used in place of an electromagnetic transducer. The weighing load causes a change of the tensile force in an oscillating string, whose change in frequency, in turn, represents a measure for the applied load. At the time of the measurement, the mechanical system of EMFC weighing cells is in an equilibrium position comparable to the equilibrium of a mechanical beam balance with counterweights. In contrast, the load-receiving portion of an oscillating-string weighing cell will slightly change its vertical position relative to the stationary portion, as the string is put in tension and thus elongated to a very small degree under the weighing load. Oscillating-string weighing cells are therefore also referred to as “small-deflection” force-measuring cells.
Both types of weighing cells are used for example in precision balances and analytical balances in the milligram range, or in microbalances in the microgram range, and need to be periodically recalibrated to maintain their capability of delivering measurement values within a prescribed tolerance range in accordance with manufacturers' specifications and regulatory requirements. These periodic calibrations are a corrective measure against factors that have an influence on the weighing cell, for example a change in the ambient temperature or barometric pressure.
The calibration is performed by periodically loading the load-receiving portion with a known weight. Based on the difference between the weight value that was determined during final inspection prior to delivery of the weighing cell and the value found in the current measurement, a correction value can be calculated by means of which the subsequent measurement results of the weighing cell can be corrected. In order to provide the most accurate calibration value possible, the calibration weight should equal the load capacity of the weighing cell. This can have the consequence that very large calibration weights will be needed.
The known state of the art includes a variety of gravimetric measuring instruments having calibration weights that are integrally incorporated.
A gravimetric measuring instrument of this type which operates according to the principle of electromagnetic force compensation and has a built-in rod-shaped calibration weight is disclosed in EP 0 955 530 B1. The rod-shaped calibration weight is arranged outside of the weighing cell and is placed on a calibration weight arm which is coupled to the load-receiving portion and serves as a force-magnifying lever. Due to this lever advantage, the mass of the calibration weight, and thus its dimensions, can be kept small. Since the calibration weight arm is always coupled to the load-receiving portion, it only performs the functions of leveraging and supporting the calibration weight during the calibration processes but is not a part of the calibration weight itself. Consequently, the calibration weight arm is part of a force-transmitting mechanism, more specifically of a lever arrangement for transmitting and leveraging the load to the measurement transducer, and remains connected to the load-receiving portion of the weighing cell also when the device operates in normal weighing mode.
As disclosed in CH 661 121 A5, the force-transmitting mechanism can also include a lever arrangement of more than one stage, wherein individual levers are suitably connected to each other by means of coupling elements, so that a force reduction is achieved between the load-receiving portion and the measurement transducer. One of the coupling elements includes holding means designed to receive a calibration weight.
A weighing cell with strain gauges which is disclosed in JP 3761792 B2 has a calibration weight with a ratio lever. A coupling element is arranged between the ratio lever and the load-receiving portion. By lifting the calibration weight and the coupling element, a load bearing which is formed on the coupling element is disengaged from a knife edge which is arranged on the load-receiving portion, whereby the ratio lever is uncoupled from the load-receiving portion.
All of the forgoing state-of-the-art solutions include calibration weight loading devices that are familiar to practitioners in the field of weighing technology.
The precise determination of the correction value is not only a function of the resolution of the measurement transducer, but also depends to a significant extent on how accurately the geometric proportions can be maintained. Even the smallest deviations of the calibration weight from its nominal position, for example on the calibration weight arm described in EP 0 955 530 B1, on the coupling member described in CH 661 121 A5, or the smallest changes in the position of the load bearing relative to the knife edge in JP 3761792 B2 cause the effective lever arm to be lengthened or shortened and thus introduce an error in the correction value. Consequently, the points of contact between the calibration weight and the calibration weight arm or between the knife edge and the load bearing are finished with the most exacting precision and thus at a high cost.